The broad, long term objective is to develop a noninvasive optical technique for imaging in vivo blood flow dynamics and tissue structure with high spatial resolution in biological systems. The specific aims are to 1) Develop a dedicated, portable high speed ODT instrument for tomographic imaging of in vivo blood flow dynamics and tissue structure in highly scattering biological tissues, 2) Develop a signal processing and image construction algorithm for high speed ODT, 3) Investigate the effects of multiple scattering and coherence gating on ODT images by simulating light absorption and scattering in turbid fluid flow media using Monte Carlo methods, 4) Image in vitro blood flow using reconstituted canine blood to verify and optimize the operation and spatial resolution of the ODT system and 5) Image in vivo blood flow in animal models and humans to demonstrate the potential clinical applications of the technique.